1. Field of the Invention
The present invention relates generally to piezoelectric resonators adapted for use in audio filter circuits of television (TV) receivers, for example, and a method of manufacturing such piezoelectric resonators. More particularly, the present invention relates to piezoelectric resonators constructed to suppress unnecessary vibrations by controlling positions of excitation-lead lines connected to excitation electrodes and a method of manufacturing such piezoelectric resonators.
2. Description of the Related Art
In audio filter circuits of TV receivers or similar devices, various types of piezoelectric resonators are used as frequency band filters.
FIG. 14 is a diagram showing a plan view of one example of such a piezoelectric resonator.
A piezoelectric resonator 51 includes a substantially rectangular elongated piezoelectric substrate 52. The piezoelectric substrate 52 is made of lead zirconate titanate piezoelectric ceramics or other suitable material and is polarized in a direction as designated by arrow "P" in FIG. 14.
The piezoelectric substrate 52 has an upper or top surface on which first and second excitation electrodes 53a, 53b are provided so as to be spaced apart from each other with a gap defined between them, and on which third and fourth excitation electrodes 54a, 54b are also provided at locations spaced by a predetermined distance from the first and second excitation electrodes 53a, 53b with a gap defined therebetween.
The first excitation electrode 53a is electrically connected via an excitation-lead line 55a to a terminal electrode 56. The second excitation electrode 53b and the third excitation electrode 54a are connected together by an excitation-lead line 55b. The fourth excitation electrode 54b is electrically connected to a terminal electrode 57 via an excitation-lead line 55c.
On the lower or bottom surface of the piezoelectric substrate 52, a common electrode 58 is arranged opposite to the first and second excitation electrodes 53a, 53b in the so-called "top/bottom-surface overlap" manner. The common electrode 58 and certain associative electrodes and lead lines to be described later have contours which are shown via dotted lines in FIG. 14. While the common electrode 58 is illustrated such that it appears larger than the region resulting from projection of the first and second excitation electrodes 53a, 53b onto the bottom surface for purposes of visual clarification of its contour shape or "phantom" depicted by dotted lines, the common electrode 58 is actually designed to have a size or configuration along the outline of the region resulting from such downward projection of the excitation electrodes 53a, 53b. Note that in the drawings of the present application, any lead lines and common electrode to be provided on the bottom surface are depicted such that the size of each is virtually larger than its real size for purposes of clarification of the outer shape illustrated via dotted lines.
The common electrode 58 is electrically connected by a ground-lead line 59a to a ground-terminal electrode 60. A ground-lead line 59b is connected to the opposite side of the ground-terminal electrode 60 in such a way that the ground-lead line 59b is connected to the second common electrode 61 which is arranged opposite to the third and fourth excitation electrodes 54a, 54b in the top/bottom-surface overlap manner.
In the piezoelectric resonator 51, the first and second excitation electrodes 53a, 53b and the associated common electrode 58 defines a first filter section and the excitation electrodes 54a, 54b and the second common electrode 61 define a second filter section. Upon application of an input signal to the terminal electrode 56, the first and second filter sections begin vibrating so that an output signal is derived from the terminal electrode 57.
Due to use of the piezoelectric effect, the first and second filter sections must be designed to vibrate in the manner described above. For other parts of the resonator 51, it is desirable that any unwanted vibration is hardly generated.
On the contrary, with the piezoelectric resonator 51, the excitation-lead line 55b and the ground-terminal electrode 60 are designed so as to overlap each other with the piezoelectric substrate 52 located therebetween. In addition, since a potential difference can occur between the excitation-lead line 55b and ground-terminal electrode 60, a part of the resonator 51 at which the excitation-lead line 55b and the ground-terminal electrode 60 are located exhibits resonance resulting in generation of unnecessary vibration components. Similarly, unnecessary vibrations can also be generated at portions where the excitation-lead line 55b and the bottom-surface ground-lead lines 59a, 59b overlap each other.
In piezoelectric resonators, it has been well known that it is desirable to arrange lead lines other than electrodes constituting the common part so that such lead lines do not overlap with the top and bottom surfaces of the piezoelectric substrate.
However, in the piezoelectric resonator 51, merely arranging the excitation-lead line 55b and ground-lead lines 59a, 59b so that they do not overlap with each other would result in an increase in unnecessary vibration. Also, in piezoelectric resonators used as band filters and having a center frequency ranging from 5 to 8 MHz, it is still difficult to sufficiently suppress the occurrence of ripples or spurious vibration components.